Heretofore, throttle bushings have been used in pumps of the volute type having a sturdy, straight shaft supported between spaced bearings. The present invention, however, is concerned with end-intake, single stage slurry pumps of the volute type and particularly high-horsepower pumps which may be connected in series and deliver slurry in the range of 600 psi and above in which the impeller of each pump has a large overhang beyond its bearings.
Where such pumps operate at high pressures, the packing gland or conventional seal on the outboard side of the bearing is always a problem, being a source of excessive maintenance and not satisfactory except for short periods of time.
While mechanical seals have been substituted for the packing gland or conventional seal, such seals are unreliable and require an undue amount of maintenance, which, due to the size of the pump, is a time-consuming job.
A throttle bushing, however, ahead of the packing gland or conventional seal, cuts down pressure as long as there is an appreciable flow through the bushing. Such bushings have been vented to atmosphere or to a controlled pressure and have been effective for multi-stage pumps having two bearings and a sturdy, straight shaft supported therebetween.
Where the impeller is relatively large and heavy and carried on the end of a large diameter impeller shaft and overhangs the impeller shaft, bearings and packing or seal a substantial amount, the shaft is given an exponential shape.
By the present invention, I utilize a relatively short packing gland, but extend the shaft from its bearing a greater distance than normal, as for example, 40 inches, and utilize a relatively long throttle bushing ahead of the packing gland. I also admit clear water under pressure through the neck of the pump casing and pass this clear water between the shaft and throttle bushing and discharge the clear water to atmosphere or a controlled pressure on the inboard side of the packing gland. A portion of the clear water is also passed into the pump to prevent the fluid pumped, which may be an abrasive slurry, from passing into the throttle bushing area. With such an arrangement, the pressure is reduced as the clear water flows through the throttle bushing to a controlled pressure, which may be low enough for conventional packing glands or mechanical seals to perform satisfactorily.
I further provide clearance between the shaft and throttle bushing and maintain this clearance in all positions the shaft will take, to accommodate the flow of clear water between the shaft and throttle bushing and to prevent seizure between the throttle bushing and shaft or scoring of the shaft by the bushing, which is usually made of a relatively hard metal. This clearance is difficult to maintain, particularly with a 1,000 pound impeller at the end of a shaft in the order of 6 inches in diameter in which the impeller is spaced in the order of 40 inches from the inboard bearing. With such an impeller and shaft, where the impeller is located in the order of 40 inches from the inboard bearing for the shaft, there will be a droop to the shaft at the impeller of substantially 0.011 inches. By uniformly increasing the clearance between the shaft and impeller, to accommodate this droop, the efficiency of the throttle bushing is decreased, even when the radial hydraulic thrust on the impeller is uniform.
It is, of course, understood that with pumps of the single volute type, the radial thrust is only uniform at the best operating efficiency of the pump and becomes non-uniform at reduced capacities, particularly when the pump is started or is stopping.
Instead of increasing the bore of the throttle bushing to accommodate the shaft droop as well as the gyrations which will occur from the impeller imbalance as the impeller wears, I match the bore of the throttle bushing to the shape the shaft will take under load, regardless of the angular direction of the load. This is done by calculating the deflection curve of the shaft for the worst operating conditions anticipated, and then machining the throttle bushing to the shape of the shaft deflection curve to assure minimum clearance and maximum effectiveness of the throttle bushing.